(1) Field of the Invention
This invention relates to an apparatus and method for CMP (Chemical Mechanical Polishing) of a semiconductor substrate and more specifically to an improved substrate carrier head for a CMP apparatus, having a substrate retaining ring that facilitates distribution of the polishing slurry onto the substrate, thus improving the polish removal rate uniformity.
(2) Description of Related Art
In the fabrication of semiconductor integrated circuits CMP (Chemical Mechanical Polishing) has been developed for providing smooth topographies on surfaces deposited on the semiconductor substrates. Rough topography results when metal conductor lines are formed over a substrate containing device circuitry. The metal conductor lines serve to interconnect discrete devices, and thus form integrated circuits. The metal conductor lines are further insulated from the next interconnection level by thin layers of insulating material and holes formed through the insulating layers provide electrical access between successive conductive interconnection layers. In such wiring processes, it is desirable that the insulating layers have a smooth surface topography, since it is difficult to lithographically image and pattern layers applied to rough surfaces. CMP can, also, be used to remove different layers of material from the surface of a semiconductor substrate. For example, following via hole formation in an insulating material layer, a metallization layer is blanket deposited and then CMP is used to produce planar metal studs embedded in the insulating material layer.
Briefly, the CMP processes involve holding and rotating a thin, flat substrate of the semiconductor material against a wetted polishing surface under controlled chemical, pressure and temperature conditions. A chemical slurry containing a polishing agent, such as alumina or silica, is used as the abrasive material. Additionally, the chemical slurry contains selected chemicals which etch various surfaces of the substrate during processing. The combination of mechanical and chemical removal of material during polishing results in superior planarization of the polished surface.
In CMP apparatuses the substrate being polished is affixed to a planar carrier head. The carrier head and affixed substrate are then pressed against the polishing pad having the polishing slurry thereon. In order to secure the substrate to the carrier head during polishing a retaining ring surrounds the substrate. The retaining ring surrounding the substrate has an inner diameter slightly larger than the diameter of the substrate. The outer diameter cf the retaining ring is approximately 1 inch larger than the inner diameter, thus forming a 1 inch wall in the retaining ring. The thickness of the retaining ring is about 7/8 inch and the upper and lower surfaces of the retaining ring are parallel. The retaining ring, as described, prevents the substrate from slipping from the polishing head during polishing, but has the detrimental effect of impeding the flow of polishing slurry to and from the substrate during polishing. It is known in the art that polishing uniformity is a function of pressure, velocity, and concentration of polishing chemicals. Furthermore, as a substrate is polished chemical byproducts locally change the uniformity of the polishing slurry through changing the slurry pH, slurry composition, and slurry particle size and these local changes can affect the local polish removal rate.
Since an important challenge in CMP is to improve the uniformity of the polish removal rate across the substrate, it is, therefore important to control the distribution of polishing slurry over the substrate during the polishing process and to control the removal of polishing byproducts from the substrate during the polishing process in order to achieve maximum polish removal uniformity. At the same time it is important to maximize the throughput of CMP machines in order to reduce the cost of product. However, state-of-the-art retaining rings for CMP apparatus impede the flow of slurry to and from the substrate during polishing and, also, impede the removal of polishing byproducts from the substrate during polishing.
U.S. Pat. No. 5,681,215 entitled "Carrier Head Design For a Chemical Mechanical Polishing Apparatus" granted Oct. 28, 1997 to Michael T. Sherwood et al describes a CMP carrier head having a retaining ring in the base assembly. The retaining ring includes a protruding portion which contacts the polishing pad and blocks the substrate from slipping out from under the base assembly. Also, described is a polishing station that includes a separate pad conditioning apparatus having a rotatable arm and an independently rotating conditioner head. The conditioner apparatus maintains the condition of the polishing pad so that it will effectively polish any substrate pressed against it while it is rotating.
U.S. Pat. No. 5,624,299 entitled "Chemical Mechanical Polishing Apparatus With Improved Carrier and Method of Use" granted Apr. 29, 1997 to Norman Shendon shows a (MP carrier head which facilitates substrate loading, retaining and unloading in a CMP apparatus.
U.S. Pat. No. 5,571,044 entitled "Wafer Holder For Semiconductor Wafer Polishing Machine" granted Nov. 5, 1996 to Hooman Bolandi et al describes a wafer holder for a polishing machine.
U.S. Pat. No. 5,643,061 entitled "Pneumatic Polishing Head For CMP Apparatus" granted Jul. 1, 1997 to Paul David Jackson et al describes a CMP polishing head which includes a retaining ring. The wafer substrate can be pressed against the polishing pad with a force independent of the force applied to the retaining ring. Spiral grooves, included in the underside of the retaining ring, assist in circulating the polishing slurry about a wafer substrate within the retaining ring.
U.S. Pat. No. 5,614,446 entitled "Holding Apparatus, A Metal Deposition System, and a Wafer Processing Method Which Preserve Topographical Marks on a Semiconductor Wafer" granted Mar. 25, 1997 to Seshadri Ramaswami et al describes a wafer retaining ring which prevents metal deposition on a portion of a wafer substrate.
U.S. Pat. No. 5,403,228 entitled "Techniques For Assembling Polishing Pads For Silicon Wafer Polishing" granted Apr. 4, 1995 to Nicholas R. Pasch describes the mounting of laminated polishing pads to a polishing platen. The mounting method prevents catastrophic delamination of the pads and, also, forms a bowl-like surface which retains slurry better than a planar surface.
The present invention is directed to a novel CMP apparatus and CMP method which improve the distribution of polishing slurry onto the substrate and improve the polish removal rate uniformity on the substrate.